During gestation, the immunologic reactivity of a female is often changed in the direction of "tolerance". This maternal immunologic unresponsiveness is demonstrable with transplanted allogeneic tissues, and is specific in that it depends upon a genetic concordance between the conceptus and subsequent tissue transplant. In nature, this phenomenon may serve to protect the fetus from the consequences of maternal immunization. We are investigating the nature and mechanisms of tolerance induction. We postulate that this information might be applied to various clinical problems, e.g., cancer in general, choriocarcinoma, certain diseases of pregnancy, and transplantation of tissues and organs. The effects of natural and induced changes in maternal reactivity on growth of the conceptus are being investigated. Studies to date show that tumor allografts, allogeneic and syngeneic conceptuses reflect different modalities of maternal reactivity.